1. Field of the Invention
This invention relates to a medical diagnostic apparatus for optically measuring the concentration of an analyte in or a property of a biological fluid; more particularly, an apparatus whose optical system includes a Fresnel reflector.
2. Description of the Related Art
A variety of medical diagnostic procedures involve tests on biological fluids, such as blood, urine, or saliva, and are based on a change in light absorption of such a fluid or an element of the fluid, such as blood serum. Some of these procedures make use of a transparent or translucent device to contain the biological fluid and a reagent. A change in light absorption of the fluid can be related to an analyte concentration in, or property of, the fluid. Typically, a light source is located adjacent to one surface of the device and a detector is adjacent to the opposite surface. Alternatively, if a reflector is located adjacent to that opposite surface, then the light can pass through the device twice and the source and detector can be on the same side of the device. A device of this latter type, in which light is first transmitted through the sample area, then reflected through a second time is called a "transflectance" device. References to "light" throughout this specification and the appended claims should be understood to include the infrared and ultraviolet spectra, as well as the visible. References to "absorption" are meant to refer to the reduction in intensity as a light beam passes through a medium, thus, it encompasses both "true" absorption and scattering.
An example of a transparent test device is described in Wells et al. WO94/02850, published on Feb. 3, 1994. Their device comprises a sealed housing, which is transparent or translucent, impervious, and rigid or semi-rigid. An assay material is contained within the housing, together with one or more assay reagents at predetermined sites. The housing is opened and the sample introduced just before conducting the assay. The combination of assay reagents and analyte in the sample results in a change in optical properties, such as color, of selected reagents at the end of the assay. The results can be read visually or with an optical instrument.
U.S. Pat. No. 4,037,974, issued Jul. 26, 1977, to T. Fletcher et al. discloses a sample cell for spectrophotometers, which prevents stray light from being carried to a detector through the cell wall. In one embodiment, the cell has a side wall whose outer surface has a sawtooth configuration, which causes light that enters the side wall from the interior of the cell either to be reflected back into the sample or pass out through the cell wall.
U.S. Pat. No. 4,116,566, issued Sep. 26, 1978, to E. Sick discloses a device for detecting defects in webs of material, using an optical system that may include a sawtooth mirror.
U.S. Pat. No. 4,233,029, issued Nov. 11, 1980, to R. Columbus discloses a liquid transport device that has opposed surfaces, each of which may have a sawtooth cross section. The sawtooth arrangement of one surface is orthogonal to the arrangement of the other surface, and the resultant liquid flow is multidirectional.
U.S. Pat. No. 5,214,277, issued May 25, 1993, to J. Drennen, III discloses a cell that holds near-infrared reflectance spectrometer samples. The spectrometer includes a light source which illuminates a sample that is contained in the cell. The cell surfaces direct the light onto the sample, collect light diffusely reflected from the sample and direct the light back toward a detector.
U.S. Pat. No. 5,522,255, issued Jun. 4, 1996, to G. Neel et al. discloses an instrument and method for determining a coagulation characteristic of blood by a reflectance technique. The sample holder includes a combination reagent heater and reflector. (See also WO95/07452.)
U.S. Pat. No. D 375,799, issued Nov. 19, 1996, to W. Leiva et al. discloses a transparent assay tester for biological fluids with a Fresnel lens.
U.S. Pat. No. 5,468,606, issued Nov. 21, 1995, to G. Bogart et al. discloses an analyte-detecting device which has a substrate with an optically active surface. When illuminated, the surface exhibits different colors, depending on whether or not the analyte is present.
U.S. Pat. No. 5,478,527, issued Dec. 26, 1995, to E. Gustafson et al. discloses a multilayer reflective biograting for use in an immunoassay. The diffraction signal from the biograting changes in the presence of an analyte